Radio access technologies for cellular mobile networks are continuously being evolved to meet future demands for higher data rates, improved coverage, and capacity. One example is the evolution of the WCDMA access technology to provide High-Speed Packet Access (HSPA). With such evolution to higher data rates, the power contributions of users in neighboring cells, which is called inter-cell interference, becomes more significant. FIG. 1 illustrates an example of a mobile radio (shown as a laptop computer) near a border between cell A and cell B. Base station A serves the mobile radio, and base station B is a non-serving base station relative to the mobile radio. As depicted with two arrows, the uplink transmission from the mobile radio is received at both base stations at about the same signal strength. In cell A, that uplink transmission is a desired signal, but in cell B, it is inter-cell interference that adversely impacts the communications quality, capacity, and throughput in cell B. To maintain communications quality, capacity, and throughput in neighboring cells, efficient and effective inter-cell interference control is needed. Inter-cell interference control is also useful for admission and congestion control as well as resource control and allocation, all of which are generally referred to as resource management. Inter-cell interference control assuming single radio frequency carrier uplink transmission is described in the commonly-assigned U.S. patent application serial number commonly-assigned U.S. patent application Ser. No. 12/192,643 entitled “Estimating and Limiting Inter-cell Interference” incorporated by reference above.
Advances in Wideband Code Division Multiple Access (WCDMA) access technology provide Multi-Carrier HSPA (High-Speed Packet Access), including Multi-Carrier HSDPA (MC HSDPA) and Multi-Carrier HSUPA (MC HSUPA) for HSPA in the downlink and uplink, respectively. Multi-carrier transmission in the uplink for example means that a mobile radio simultaneously transmits on multiple radio frequency carriers which permits the mobile radio to achieve very high data rates. Higher data rates require larger transmission power which creates higher inter-cell interference to users in the neighboring cells.
In order to maximize coverage, a mobile radio (often referred to as user equipment (UE) in WCDMA) may need to change from multi-carrier transmission to single-carrier transmission on a “primary carrier” or “anchor carrier.” For this purpose, and in order to simplify mobility procedures, selection of a serving cell and an active set for a UE may be based on channel quality measurements on the primary carrier only. But assuming there is adequate coverage and multi-carrier transmission where the UE is transmitting on the primary and one or more secondary carriers simultaneously, if a strong neighbor cell is missing in the UE's active set on a secondary carrier (no soft handover with this neighbor cell), UE transmissions on that secondary carrier in the serving cell may cause significant inter-cell interference in the neighboring cell. To avoid this problem and maintain system performance, coverage, and stability, some sort of inter-cell interference control in a MC-HSUPA system is therefore needed.
Multi-carrier operation in HSDPA, also known as Dual-cell HSDPA, has been a 3GPP Rel-8 work item to support multi-carrier transmission in downlink. To support higher peak rates and to increase cell and system throughput in uplink, multi-carrier operation in uplink transmission has been initiated. As a baseline, a MC-HSUPA system includes multiple legacy radio frequency carriers on which an MC-HSUPA capable UE can transmit simultaneously. The cell's carriers are assumed to be managed by the same base station (often referred to as a Node-B). Scheduling per carrier is presumed to be performed according to baseline HSUPA or Enhanced Uplink EUL.
For the uplink of a WCDMA system, the common resource shared among the terminals is the amount of tolerable interference, i.e., the total received power at the base station. The amount of common uplink resources a UE uses depends on the data rate (e.g., the transport format) at which the UE is transmitting. Generally, higher data rates require larger transmission power, and thus, the resource consumption is higher.
Scheduling, which in HSUPA is handled by the Node-B, is the mechanism determining when a certain UE is allowed to transmit and, if transmission is allowed, at what to maximum data rate. With scheduling, the Node B decides the UE's Transport Format Combination (TFC) selection for the Enhanced Dedicated Channel (E-DCH). The E-DCH scheduling framework is based on scheduling requests sent by the UEs to request resources and scheduling grants sent by the Node B scheduler to control the UE transmission activity.
Two types of scheduling grants are used: absolute grants and relative grants. The absolute grants set an upper power limit the UE may use for data transmission, which determines the maximum data rate. The relative grants update the resource allocation for a UE using one of three values: ‘up’, ‘down’, or ‘hold’, thereby instructing the terminal to increase, decrease, or not change transmit power relative to its current transmit power. Absolute grants can only be transmitted from the E-DCH serving cell, while relative grants can be transmitted from both the serving and non-serving cells. So one way the non-serving cell can control inter-cell interference is by sending relative grants to the UE.
A scheduling grant controls a limit of the E-DPDCH/DPCCH power ratio the UE may use. In the above-identified, commonly-assigned application, a E-TFC selection algorithm is used limit the maximum E-DCH data rate. Because only the UE is aware of the power and buffer situation at the time of transmission, the Node B scheduler can only set an upper limit of the power ratio beyond which the UE is not allowed to transmit. The UE selects a suitable E-TFC based on available UE power. E-DCH users can ask for a higher data rate by sending rate request or setting a “happy bit” to “unhappy.”
Mobility management and handover control are performed in the Radio Network Controller (RNC) in WCDMA. In soft handover, a UE maintains radio connections with more than one base station or cell on the same WCDMA radio frequency carrier. All the cells having radio connections with a UE form the UE's active set. To establish a soft handover, radio connection mobility measurements, such as measuring a base station's pilot channel CPICH signal to interference radio, Ec/Io, are carried out at UE/mobile terminal and reported to the RNC.
Establishing soft handover radio connections by including neighbor cells into active set is one way to mitigate inter-cell interference. But soft handover relies on the UE mobility measurements to be reported to RNC which is a relatively slow procedure and thus not optimal from a performance point of view. Moreover, a large number of UEs needs to be in soft handover for this method to be sufficient to control inter-cell interference as higher rates are introduced in a system, A too-large soft handover region might be problematic—not only due to the high RBS hardware, RNC hardware, and Iub transmission link capacity required—but also due to poor signal-to-interference (SIR) ratio in downlink for radio links in non-serving cells having a low path gain relative to the serving cell. In addition, when more than one radio frequency carrier is used for a UE connection, the number of feasible carrier combinations and potential serving/non-serving cells is complicated and complex, Moreover, if a secondary carrier is temporarily deactivated by the Node-B controlling the serving cell, and thus mobility measurements are not available, then when the secondary carrier is reactivated, there will be an additional delay until the active set is updated. As a result, mobility measurements on multi-carriers may not always be available in RNC.
Thus, soft handover and mobility control may not be an optimal approach for controlling inter-cell interference in a multi-carrier uplink transmission system. Moreover, when the mobility control is performed only on one carrier, such as the primary carrier, there are several scenarios where inter-cell interference on one or more secondary carrier(s) can be a problem. One scenario is irregular deployment of multi-carrier base stations, where some base stations can support multi-carriers while others can only support a single carrier. If a multi-carrier UE moves towards a neighbor cell where the primary carrier being used by the UE is not deployed and a secondary carrier being used by the UE is the only carrier in that neighbor cell, then soft handover to the neighbor cell is not possible on the primary carrier. As a consequence, the UE's transmission can cause excessive interference to the neighbor cell on the secondary carrier. Another irregular deployment case is when micro/pico/femto-cells are embedded on one (or multiple) carrier frequencies. If a multi-carrier mobile terminal has one of those micro/pico/femto-cells as a strong neighbor cell on the secondary cannier, and the strong neighbor cell cannot be included in the active set because the mobility control is based on the primary carrier, then the result is as just described where the multi-carrier user may cause excessive interference to UEs in those micro/pico/femto-cells.
Thus, there is a need for inter-cell interference control multi-carrier HSUPA systems on one or more secondary carriers.